Diffractive optics are used in a wide variety of applications. In some applications, diffractive optical elements (DOEs) are used in creating a desired projection pattern, for purposes such as optical three-dimensional (3D) mapping, area illumination, and LCD backlighting. DOE-based projector designs are described, for example, in U.S. Patent Application Publication 2009/0185274, whose disclosure is incorporated herein by reference.
The “efficiency” of a DOE is a measure of the amount of input energy that the DOE diffracts, in relation to the energy of the incoming beam. This efficiency can vary in production due to manufacturing tolerances. It can also change during the lifetime and operation of the DOE for various reasons. For example, humidity and other vapors can condense on the DOE surface and lower its efficiency, or excess heat, due to a malfunction or misuse, can deform the DOE and change its efficiency. Such changes in efficiency can result in undesirable increases in the intensity of the zero diffraction order, which is not diffracted by the projection optics and may thus continue straight through the DOE to the projection volume.
U.S. Pat. No. 8,492,696, whose disclosure is incorporated herein by reference, describes a DOE-based projector with a built-in beam monitor, in the form of an integral optical detector. The detector signal can be continuously or intermittently monitored by a controller in order to evaluate the DOE efficiency and inhibit operation of the projector if the signal is outside a certain range.